Work proposed here will likely yield insight into the general mechanisms underlying muscle development and differentiation. A critical feature of muscle development is interaction with the developing nervous system. Any advance in the understanding of the normal interdependence between developing muscles and nerves might ultimately lead to progress in the study of human neuromuscular disease. The experimental model in this study will be the fruitless (fru) gene in Drosophila melanogaster. Mutation of this gene has suggested probable roles for a fru gene product in: (a) the developing nervous system--mutant adult males are unable to process courtship-stimulating cues properly leading to their aberrant courtship of other males; and (b) muscle development--mutant adult males fail to develop a large, paired muscle which normally appears in the fifth abdominal segment (the "Muscle of Lawrence"). Unraveling these pleiotropic effects of fru during fly development hinges on its molecular cloning and identification of its product(s). The starting point for experiments designed to achieve this goal will be an identified cDNA which maps to the fru cytogenetic location. This cDNA and related probes will be used to identify genomic sequences relevant to fru, and to provide data on the expression pattern of fru during the life of the fly. The next-- and most important--goal will be to identify those sequences within fru which play a role in adult male muscle differentiation. Here, a unique approach is proposed. Many Drosophila species fail to differentiate the Muscle of Lawrence in adult males as the norm--this includes species belonging even to the same species subgroup as melanogaster. Thus the role of fru as a regulator of muscle development may be a "gain of function," or genetic addition, to this gene as it exists in D. melanogaster. If so, such sequences, whether regulatory or protein- encoding, will be identified by subtractive hybridization methods. Finally, the mechanism by which fru serves as a regulator of muscle differentiation may be evolutionarily conserved. To test this possibility, fru-homologous sequences will be screened for in murine genomic and embryonic cDNA libraries.